1. Field of the Invention
Techniques for establishing a connection between the central office in a telephone network (or other switching system such as a local PBX) and a particularly station set are, of course, well known. A telephone line consisting of a pair of conductors, historically referred to as tip and ring, connects the central office to a particular station set. The central office equipment supplies an operating line voltage, typically a nominal -48 or -24 volts, across the tip and ring pair and monitors the line to detect when the station set has "seized" the line (i.e. someone has taken the station set receiver "off-hook"). Because the distance from central offices to particular station sets will vary greatly, central offices monitor the current flow in a telephone line, and line seizure is established by the station set connecting an impedance between tip and ring to establish a loop. When a call is waiting for a particular station set, the central office will vary the potential between tip and ring in a periodic fashion to generate a ring signal. This ring signal is detected in the station set which typically then generates an audible signal to indicate a call.
This simple system has worked well for decades in the telephone network to connect single station sets to central offices, or other switching systems through a single line, but presents problems in connecting a number of lines to one or more station sets. Such installations are typically referred to as key set equipment, or "1A2" equipment, and provide the well known "push button telephone" and a central control connected to each individual station set by a multiconductor cable. This central control responds to signals from the station set to activate one of a number of telephone lines connected to the station set, and also to place selected lines on hold and provide line status indicator signals to control indicator lamps associated with each telephone line at the station set. Such central control and system wiring for such key set equipment is complex and costly; particularly for local systems involving only a small number of outside telephone lines and a small number of station sets. Further, each station set in such key set equipment is usually provided with the capacity for connection to the maximum number of telephone lines which can be controlled by the central control.
A recent solution to this problem has been provided by digital telephone switching equipment. Station sets connected to digital switching equipment typically are connected by one line to provide a voice signal path and a second, digital line which carries control signals between the switching equipment and the station set. Such digital switching equipment allows a great variety of sophisticated telephone functions, such as call forwarding, and is undoubtedly the direction in which the technology may be expected to move in the future. However, due to the enormous capital base invested in conventional tip and ring telephones, it is expected that for the foreseeable future, the ability to provide multi-line connection to conventional tip and ring telephone lines will be necessary.
It should also be noted that digital technology has been incorporated into station sets intended for use with conventional tip and ring telephone lines. Such station sets, sometimes referred to as "smart telephones" incorporate a microprocessor for control. A typical such phone is marketed by the assignee of the subject invention under the trademark "Connexions", and is described in commonly assigned U.S. patent application Ser. No. 895,017. Such "smart telephones" provide a number of input controls, in addition to the typical telephone keypad used for dialing. The microprocessor responds to these controls to implement such features as automatic dialing.
U.S. Pat. No. 4,490,583, to: Bednarz et al. is one proposed apparatus for the connection of a number of telephone lines to a single line tip and ring station set. The apparatus of Bednarz et al., however, is connected to the station set only through the tip and ring pair, and accordingly is not capable of taking advantage of the capabilities of modern "smart telephones". Thus, the apparatus of Bednarz et al. includes mechanically operated switch closures for selecting a particular telephone line and indicator lamps to indicate the status of each line. Further, the apparatus of Bednarz et al. is not capable of distinguishing a telephone line in a hold condition from a telephone line in a seized condition, and each controller as taught by Bednarz et al. must be separately connected to all other controllers to determine the status of the telephone lines.
Because the status of the lines in systems such as that taught in Bednarz and "keyset" equipment is communicated over separate lines, such systems will not release a hold condition on a line if the line is picked up by a conventional single line telephone; making the use of conventional single line telephones with such systems awkward and less than fully satisfactory to some users.
Thus, it is an object of the subject invention to provide a means for connection of multiple telephone lines to a single station set which does not require a central controller of the type usually used with key set equipment.
It is still another object of the subject invention to provide an apparatus for the connection of multiple telephone lines where a number of such apparatus maybe used with a corresponding number of station sets to connect a group of telephone lines to each of those stations sets without the need to interconnect the connection apparatus, and which is also suitable for use with conventional single line telephones.